Sub-cooled pipe line for removal of liquid from refrigerated storage tank



Jan. 23, 1968 O w, JOHNSON 3,364,689

SUB-COOLED PIPE LINE FOR REMOVAL OF LIQUID FROM REFRIGERATED STORAGE TANK Filed June 30, 1966 El HZ cuuME i H i 0 AZ g I Z 35 l n 1 30 52 ll 'I l g .lI

f g VACUUM I N VENTOR. Our/[e h/ Joy/wow United States Patent SUB-CGOLED PEPE LINE FOR REMOVAL OF LIQUID FRQM REFRTGERATED STGRAGE TANK Oliver W. Johnson, Palo Alto, Caiiii, assignor to Chicago Bridge & Iron Company, Oak Brook, EL, :1 corporation of Illinois Filed June 30, 1966, Ser. No. 561,819 3 Claims. (Cl. 62-45) ABSTRACT OF THE DISCLOSURE In a liquified gas storage system in which liquified gas is withdrawn from a storage tank through a suction pipe line, there is provided a cooling pipe line in thermal conductive relation with the suction pipe line, and a vacuum pump connected to the cooling pipe line for reducing the pressure within the cooling pipe line to cause sub-cooling of the liquified gas in the suction pipe line.

This invention relates to liquified gas storage and removal apparatus and more particularly to the refrigeration of a suction pipe line between a storage tank and a suction pump during removal or discharge of a liquified gas from the tank.

Ammonia, methane, propane, oxygen, nitrogen, hydrogen, and other materials, which are normally gases at ambient temperatures and pressures are often liquified and stored in refrigerated storage tanks at atmospheric, or slightly higher pressure. To remove a liquified gas from the tank a suction pump withdraws the liquid through a suction line connected to the tank. In the desired manner of operation, starting of the pump fills the suction line with liquified gas which vaporizes just enough to cool the line and keep it cold so as to enable the pump to continue to remove the liquid from the tank.

In some instances, such as where the storage tank is a flat bottom tank and the suction pump is thus operating under a low liquid head, the liquid in the suction line continues to vaporize because of heat gain from the surrounding environment, and the pump must then start on a mixed liquid and vapor phase suction. If the suction line is long, it may be impossible to get the suction pump to take liquid suction. The pump then loses whatever liquid priming there may have been and cavitates, which can cause damage to certain pump seals. Furthermore, since suction pumps utilized for this purpose are constructed such that the pump bearings are lubricated and cooled by the liquified gas being pumped, a failure to take liquid suction with the pump operating can severely damage the pump bearings.

To alleviate this problem, separators have been provided at the pump to gravity separate the liquid from the vapor, and enable the pump to take liquid suction. Generally, these methods are quite useful for short suction lines, but have not been used extensively for relatively long lines.

According to the present invention the suction pipe line connecting a liquified gas storage tank with a suction pump is sub-cooled to maintain the liquified gas within the suction pipe line in the liquified state.

In one aspect of the invention a cooling pipe line extends integrally with the suction pipe line and means are provided for conveying liquified gas from the suction line to within the cooling line. A vacuum pump is coupled to the cooling pipe line for reducing the pressure within a ainst this line which vaporizes liquified gas inside and thereby provides sub-cooling of the liquified gas in the suction line.

The invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a diagrammatic view illustrating a liquified gas storage system including a sub-cooled suction line constructed in accordance with the principles of the present invention;

FIGURE 2 is a fragmented view illustrating an alternative embodiment of this invention; and

FIGURE 3 is a sectional view illustrating another alternative embodiment of a sub-cooled suction line according to the present invention.

Referring now to FIGURE 1 there is illustrated a liquified gas storage system 19 including a storage tank 12 for storing liquified gases such as ammonia, methane, propane, oxygen, nitrogen, hydrogen, and others at approximately atmospheric pressure. The storage tank illustrated in FIGURE 1 is the type known as a hat bottom which rests on the ground 14 and has an outlet 16 near the bottom of the tank. A suction line 1-8 is connected at one end thereof to a suction pump 20 which is used for discharging the liquid from the tank.

The storage tank 12 is generally formed of double walled construction with the suction line 18 leading to the liquified gas within the inner Wall of the storage tank in a known manner. Also, the suction line 18 contains suitable expansion bellows (not shown) to allow for expansion and contraction of the suction line with variations in temperature.

The suction pump 20 is mounted by suitable means Within the ground 14 some distance from the storage tank 12 such that a Well portion 22 of the pump 20 extends for approximately six feet below the suction line 18. An output end 24 of the suction line 18 is connected to an input port 26 of the pump 2 so that the pump draws liquified gas from the storage tank 12 through the suction line 18, output end 24, input port 26, down into the well portion 22, and back up to a discharge head 28. The liquified gas is then pumped through an output port 30 and a connecting output line 32 to the desired location. The system includes a shut-off valve 34 for controlling the flow of liquified gas from the storage tank.

The system as heretofore described is a type of arrangement currently used for withdrawing liquid from the storage tank. The shut-off or block valve 34 is opened, prior to starting the pump 26. The suction line 18 then fills with liquified gas which vaporizes enough to cool the line 18 and keep it cold. However, under certain conditions such as where the liquid in the suction line 18 may be some degrees above the atmospheric boiling point (that is, the particular pressure and temperature above which the liquified gas is converted to a vapor) because of the low liquid head in the tank, vaporization continues due to heat gain from ambient conditions which then makes it necessary for the pump 20 to start on a mixed vapor and liquid phase suction. This undesired vaporization of the liquified gas in the suction line particularly occurs in installations where the pump 20 is located at a relatively long distance from the storage tank 12. Under such conditions the pump 20 cannot obtain liquid suction and damage to the pump seals and bearings could result.

According 0 one embodiment of the present invention, as illustrated in FEGURE 1, a sub-cooling pipe line 36 has one portion located within the suction line 18 with an end 38 near the shut-elf valve 34 at the storage tank 12, and extending substantially along the en'ire length of the suction line 18 towards the suction pump 21 The subcooling line as protrudes through the suction line 18 as indicated by reference character 4x) in a suitably sealed manner and is coupled to pressure reducing means such as a vacuum pump 42. The sub-cooling line end 38 is closed ofig'" except for an orifice 44 which restricts the passage of liquified gas from within the suction line 18 to within the cooling line 36.

The vacuum pump 42 reduces the pressure inside the subcooling line 36 such that the small amount of liquified gas passing from the suction line 13 into the subcooling line 36 through the orifice 44 will vaporize along the length of the cooling line due to the reduced pressure. Vaporization of the liquified gas within the sub-cooling line 36 provides refrigeration to substantially sub-cool the suction line 18 to more than overcome the heat gain of the suction system due to ambient conditions. This enables the pump 20 to start on liquid phase suction with vaporization of the liquified gas occurring only in the subcooling line 36. The degree of cooling will depend on the amount of time allowed for vaporization to occur in the sub-cooling line and the volume of liquified gas evaporated or boiled therein.

This of course depends on the capacity of the vacuum pump 42 and the size of the orifice 44. In the desired mode of operation, sub-cooling of the liquified gas in the suction line 18 will provide a volume of liquified gas sufficient to insure that the pump 20 Will take suction promptly, and that continued cooling of the suction line will not be required after normal flow has been established. As a prac'ical matter, the vacuum pump 42 should be of the type which will not be damaged by wet suction, or a knock-out drum or heat exchanger could be provided.

Normal support means such as a spacer having spokes extending between the inner sub-cooling line 36 and the outer suction line 18 can be provided to maintain the subcooling line in position without obstructing the flow of liquified gas. In order to reduce heat gain of the suction system, the tank connection, shut-oii valve, suction pipe and pump should be insulated.

It must be understood that i is within the teachings of the present invention to provide means within the subcooling line 3% to improve heat transfer between the subcooling line and the suction line. For insance, the subcooling line could be provided externally with longitudinal extending fins which would improve heat transfer without seriously impeding the liquid flow to the suction pump. It is of course Well Within the art to determine the size of the orifice 44 or the number of such orifices and the capacity of the vacuum pump 42 required to provide refrigeration and sub-cooling of a sufficient amount to more than overcome the normal heat gain to the enire suction system.

Referring now to FIGURE 2 there is illustrated an alternative embodiment of a subcooling suction line according to the principles of the present invenion. In this embodiment, the suction line 18 surrounds a sub-cooling line 46. The sub-cooling line 46 has one end 48 which is entirely or substantially open and another end which is connected to a vacuum pump, such as pump The subcooling line end 48 is located closely adjacent to the storage tank 12 as in the manner of the end 38 illustrated in FIGURE 1. At or near this end of the sub-cooling line, there is provided a pressure reducing valve 50. Upon operation of the vacuum pump 42 the pressure reducing valve 50 opens and closes to interrupt the flow of liquified gas within the sub-cooling line 46 so as to maintain a definite level of reduced pressure inside this line. Therefore the refrigeraing effect produced by the vaporization of the liquified gas in the sub-cooling line 46 can be varied by varying the fiow through the vacuum pump 42 while maintaining a constant dilierential pressure on both sides of the valve 53.

Another alternative embodiment to the present invention is illustrated in FIGURE 3 wherein a suction line 52 is surrounded by a sub-cooling line 54 having at one end thereof a port 56 which is connected to a suitable vacuum pump. Suitable expansion bellows 58 are provided for expansion and contraction of the sub-cooling line 54. End 60 of the subcooling line 54 is located close to the storage tank 12, and it may be noted from FIGURE 3 that the inner suction line 52 has been provided With a series of orifices 62 at this end. Thus, upon actuation of the vacuum pump 42 liquified gas from within the storage tank 12 passing from the suction line 52, through the orifices 62 and into the sub'cooling line 54 will be vaporized due to the reduced pressure within the sub-cooling line to refrigerate the surrounded suction line 52. In this embodiment of the invention it may be noted that the sub-cooling line surrounds the suction line 52. The suction line is in efiect insulated by the sub-cooling line so that the suction line is not affected by ambient conditions.

Instead of locating the orifices 52 directly in the suction line 52, other arrangements within the skill of the art are readily available. For instance it is wi hin the teachings of this invention to provide an arrangement wherein the liquified gas from the tank is divided into a normal, non apertured suction line passing through a sub-cooling line, and a valve controlled short bypass line branching from the suction line near the tank to present a controlled flow of liquified gas to the sub-cooling line. When conditions occasionally enable proper pumping without sub-cooling, the bypass valve can be shut off if desired to eliminate the unnecessary flow of liquified gas through the bypass and sub-cooling line. This would provide a flexible pump system capable of operating with or wi hout sub-cooling.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is: I

1. In a liquified gas storage system having a storage tank for storing a liquified gas, a suction pump for removing the liquified gas from said tank, and a suction pipe line interconnecting said tank with said pump, the improvement comprising:

a cooling pipe line Within said suction pipe line ex tending substantially along the length of said suction pipe line from one end near said tank to the other end near said suction pump, said cooling pipe line having an orifice at the end nearest the tank for enabling the liquified gas within the suction pipe line to flow into the cooling pipe line, and a vacuum pump connected to said cooling pipe line at the end op posite the orifice to reduce the pressure Within the cooling line and enable the liquified gas passing through said orifice to boil or vaporize at the reduced pressure as it travels towards the vacuum pump and thereby sub-cool the surrounding liquified gas along the length of said suction pipe line.

2. In a liquified gas storage system as claimed in claim 1, the improvement further including valve means connected to said cooling pipe line between said orifice and said vacuum pump for regulating the amount of liquified gas conveyed from said suction pipe line to said cooling pipe line to thereby vary the sub-cooling of the liquified gas within said suction pipe line.

3. In a liquified gas storage system having a storage tank for storing the liquified gas, a suction pump for removing the liquified gas from said tank, and a suction pipe line interconnecting said tank with said pump, the improvement comprising:

a cooling pipe line surrounding said suction pipe line and extendin substantially along the length of said suction pipe line from one end near said tank to the other end near said suction pump, said suction pipe line having an orifice at the end nearest the tank for enabling the liquified gas Within the suction pipe line to flow into the cooling pipe line, and a vacuum pump connected to said cooling pipe line at the end opposite the orifice in said suction pipe line to reduce the pressure within the cooling line and enable the liquified gas passing through said orifice to boil or vaporize at the reduced pressure as it travels towards the vacuum pump and thereby sub-cool the surrounding liquified gas along the length of said suction pipe line.

References Cited UNTTED STATES PATENTS LLOYD L. KING, Primary Examiner. 

